1. Field of the Invention
The present invention relates to a quartz crystal oscillator of a surface-mount type, and more particularly, to a quartz crystal oscillator which prevents fluctuations in frequency when an impact is applied thereto.
2. Description of the Related Art
A surface mounting crystal oscillator is used mainly as a frequency reference source particularly for a variety of portable electronic devices such as portable telephones because of its compact size and light weight. In recent years, an increasingly strict tolerance is imposed for a frequency deviation of an oscillation frequency in a surface mounting crystal oscillator. For example, a crystal oscillator for a portable telephone is required to limit a frequency variation ratio with respect to a reference frequency (nominal frequency), for example, within xc2x11 ppm.
FIG. 1 is a cross-sectional view illustrating an exemplary configuration of a conventional surface mounting crystal oscillator. As disclosed, for example, in Japanese Patent No. 2969526 (JP, 2969526, B), the illustrated crystal oscillator comprises IC (integrated circuit) chip 2 and quartz crystal blank 3 contained in container body 1, and metal cover 4 overlaid to hermetically encapsulate these components. Container body 1, which is made of laminated ceramics, is formed with a recess on its top surface, and the recess has steps on its inner wall. On the bottom and side surfaces of container body 1, which define outer surfaces thereof, mounting electrodes 5 are formed each for surface mounting. Mounting electrodes 5 are used to connect the crystal oscillator to an external circuit which is provided on a printed wiring board when the crystal oscillator is mounted on the printed wiring board.
IC chip 2 has an oscillator circuit integrated therein, which utilizes a quartz crystal unit comprised of crystal blank 3. FIG. 2 illustrates the circuit arrangement of the crystal oscillator. Oscillation capacitors Ca, Cb are connected respectively to one and the other ends of crystal unit 3A comprised of crystal blank 3 for making up a resonance circuit. These capacitors Ca, Cb have their other ends connected to a ground potential. An input and an output terminal of oscillation amplifier 6 are connected respectively to one and the other ends of crystal unit 3A for amplifying and feeding back an oscillation frequency which depends on the resonance circuit. Feedback resistor 7 is also connected between the input and output terminals of oscillation amplifier 6. Within the circuit indicated by solid lines in FIG. 2, portions except for crystal unit 3A are formed in IC chip 2.
Although not shown in FIG. 2, a pair of connection terminals for use in connection with crystal unit 3A, a power supply terminal, an output terminal, and a ground terminal are provided on one major surface of IC chip 2. Corresponding to these terminals, a circuit pattern is formed on the bottom of the recess in container body 1. Then, the one major surface of IC chip 2, on which the respective terminals are exposed, is secured on the bottom of the recess in container body 1 by facedown bonding. The facedown bonding involves, for example, providing a gold (Au) bump on each of the terminals exposed on the one major surface of IC chip 2, and bonding the gold bumps with the circuit pattern on container body 1 by ultrasonic thermocompression bonding. Container body 1 is provided with via-holes and the like for electrically connecting the circuit pattern on the bottom of the recess with mounting electrodes 5, such that the power supply terminal, output terminal, and ground terminal of IC chip 2 are electrically connected to mounting electrodes 5 by the facedown bonding.
Crystal blank 3 is, for example, an AT-cut quartz crystal blank, and is provided with a pair of excitation electrodes (not shown) on both major surfaces, and extending electrodes are extended from these excitation electrodes toward both ends of one side of crystal blank 3. Then, crystal blank 3 is held within the recess by securing leading ends of both extending electrodes to the steps formed on the inner wall of container body 1 with conductive adhesive 8. In this event, the other end of crystal blank 3 is carried on the step in contact with or in close proximity to the same. A pair of extending electrodes of crystal blank 3 are connected to the pair of connection terminals of IC chip 2 through the circuit pattern formed within the recess of container body 1. Metal cover 4 is bonded to metal ring 9 disposed on the top surface of a side wall of container body 1 by seam welding to seal the opening face of the recess.
In the crystal oscillator configured as described above, IC chip 2 and crystal blank 3 are accommodated in a single recess of container body 1, so that the crystal oscillator can be reduced particularly in height, as compared with a crystal oscillator which has recesses in both major surfaces of a container such that a crystal blank is hermetically accommodated in the recess in one major surface while an IC chip is accommodated in the recess in the other major surface. When the container body is formed with recesses on both major surfaces, the resulting container body has an H-shaped cross-section. Since the container body is required to withstand a force applied thereto when the IC chip is secured by compression bonding, the strength of the container body must be enhanced by increasing the thickness of a horizontal part in the H-shaped cross-section, resulting in a correspondingly increased height of the container body.
However, the conventional surface mounting crystal oscillator as illustrated in FIG. 1 has a problem that the frequency fluctuates when a drop impact is applied to the crystal oscillator. Specifically, when a finished crystal oscillator is applied with an impact in a drop impact test by dropping the same to a concrete floor from 1.5 meters above, the oscillation frequency fluctuates in the positive direction, resulting in a failure in satisfying a regulation against the oscillation frequency which stipulates, for example, that a deviation xcex94f/f shall be within 1 ppm, where f is a nominal oscillation frequency, and xcex94f is a frequency changing amount from the nominal oscillation frequency.
The drop impact test herein referred to involves dropping a surface mounting crystal oscillator twice in positive and negative directions, respectively, with respect to three-dimensional coordinate axis directions (X-, Y-, Z-directions) in one cycle as defined herein, and recording a frequency change (xcex94f/f) in each cycle. Here, the test is conducted in ten cycles. FIG. 3 shows the result of the drop impact test conducted for a conventional surface mounting crystal oscillator, the oscillation frequency (nominal frequency) f of which is 14.4 MHz. Ten crystal oscillators were tested, and FIG. 3 shows frequency changes in each cycle for the one which presented the largest frequency change and the one which presented the smallest frequency change, respectively.
It is therefore an object of the present invention to provide a surface mounting quartz crystal oscillator which maintains a small height, reduces a frequency change caused by an impact, and has a small frequency deviation and a small frequency changing amount.
The present inventors diligently made investigations on fluctuations in frequency caused by a drop impact as described below to complete the present invention. Specifically, the object of the present invention is achieved by a surface mounting crystal oscillator which includes a container body having a recess, a crystal blank having one end held on a step formed on an inner wall of the recess and functioning as a crystal unit vibrator, an IC chip secured on the bottom of the recess, and a metal cover for sealing an opening face of the recess. The IC chip has integrated therein a first and a second capacitor connected to one and the other ends of the crystal unit, respectively, to form a resonance circuit, and an oscillation amplifying element for amplifying an oscillation frequency depending on the resonance circuit and feeding back the amplified oscillation frequency. A first stray capacitance produced between the crystal blank and IC chip is connected in parallel with the first capacitor, while a second stray capacitance produced between the crystal blank and metal cover is connected in parallel with the second capacitor. A first gap defined between the crystal blank and IC chip, and a second gap defined between the crystal blank and metal cover are set in accordance with a changing amount of the oscillation frequency due to a change in the first stray capacitance and the second stray capacitance in a direction in which a change in an equivalent series capacitance is reduced as viewed from the crystal unit, while maintaining a gap between the IC chip and metal cover constant.
Next, description will be made on the result of the investigations made by the present inventors for achieving the object of the present invention.
In the surface mounting crystal oscillator in the configuration illustrated in FIG. 1, IC chip 2 and metal cover 4 are positioned on the upper and lower sides of crystal blank 3, respectively. Then, a metal conductor, i.e., a chip conductor is formed on a second major surface of IC chip 2, i.e., the surface opposing crystal blank 3. This chip conductor is grounded to a ground potential together with metal cover 4. Therefore, as indicated by broken lines in the equivalent circuit of FIG. 2, stray capacitances C1, C2 are produced between the excitation electrodes on both major surfaces of crystal blank 3, i.e., both terminals of crystal unit 3A in the circuit diagram, and the chip conductor and metal cover 4, respectively. These stray capacitances C1, C2 are connected in parallel with the aforementioned oscillation capacitors Ca, Cb, respectively. In FIG. 1, stray capacitance C1 is connected to capacitor Ca and stray capacitance C2 to capacitor Cb. However, if crystal blank 3 is secured to container body 1 upside down, stray capacitance C2 is connected to capacitor Ca and stray capacitance C1 to capacitor Cb.
In the configuration illustrated in FIG. 1, the other end of crystal blank 3 is not secured and therefore functions as a free end with respect to the fixed one end, i.e., the end which is secured to the step formed on the recess. If the surface mounting crystal oscillator is dropped and receives an impact under this situation, the other end of crystal blank 3 swings up and down about the fixed one end. However, since the other end of crystal blank 3 is in contact with or in close proximity to the step formed on the recess of container body 1, its downward swinging motions or movements are limited by the step. This also causes a stress in conductive adhesive 8 which fixes the one end of crystal blank 3. Then, conductive adhesive 8 is changed in state into deformation, with the result that crystal blank 3 is inclined, for example, diagonally upward toward the other end and held in this inclined state. In other words, the other end of crystal blank 3 is lifted up.
As the other end of crystal blank 3 is lifted up, substantial gaps d1, d2 change between crystal blank 3 and IC chip 2 and between crystal blank 3 and metal cover 4, respectively, causing an associated change in stray capacitances C1, C2. In this event, stray capacitance C1 changes in a decreasing direction since gap d1 is increased, while stray capacitance C2 changes in an increasing direction since gap d2 is reduced. As is well known, stray capacitances C1, C2 are determined by:
C1=∈S1/d1;
and
C2=∈S2/d2,
where ∈ is a dielectric constant of the gaps; S1 is the area of the excitation electrode of crystal blank 3 opposing IC chip 2; and S2 is the area of the excitation electrode of crystal blank 3 opposing metal cover 4.
Since stray capacitances C1, C2 are connected in parallel with the aforementioned oscillation capacitors Ca, Cb, respectively, a change in stray capacitances C1, C2 results in associated fluctuations in the oscillation frequency. The oscillation frequency of a quartz crystal oscillator depends on an equivalent series capacitance, when the circuit is viewed from crystal unit 3A, and becomes higher as the equivalent series capacitance is smaller. Equivalent series capacitance CQ in this event is expressed by equation (1):                                                         CQ              =                              xe2x80x83                            ⁢                                                (                                      Ca                    +                    C1                                    )                                ⁢                                                      (                                          Cb                      +                      C2                                        )                                    /                                      {                                                                  (                                                  Ca                          +                          C1                                                )                                            +                                              (                                                  Cb                          +                          C2                                                )                                                              }                                                                                                                          =                              xe2x80x83                            ⁢                              CxCy                /                                  (                                      Cx                    +                    Cy                                    )                                                                                        (        1        )            
where Cx is a combined capacitance of Ca and C1, and Cy is a combined capacitance of Cb and C2. Strictly, input and output capacitances of oscillation amplifier 6 should be taken into account, but they are omitted for convenience because the description will not be affected by them.
The following description will be made on investigations on individual cases based on the quantitative relationships between oscillation capacitances Ca, Cb and between stray capacitances C1, C2, and the like.
Case 1
First, assuming that oscillation capacitances Ca, Cb have the same value (Ca=Cb), and gaps d1, d2 are equal (d1=d2) to each other, i.e., initial stray capacitances C1, C2 are equal (C1=C2) before a drop impact. Therefore, combined capacitances Cx, Cy are also equal. As described above, the other end of crystal blank 3 is lifted up after the drop impact to increase substantial gap d1, so that stray capacitance C1 is reduced after the drop impact. This changing amount of stray capacitance C1 is represented by xcex94C1. On the contrary, stray capacitance C2 is increased because gap d2 is narrowed down. This changing amount of stray capacitance C2 is represented by xcex94C2.
In this event, since capacitances of oscillation capacitors Ca, Cb are equal, and initial stray capacitances C1, C2 are also equal, changing amounts xcex94C1, xcex94C2 of stray capacitances C1, C2 in decreasing and increasing directions are substantially equal, while changing amounts xcex94Cx, xcex94Cy of combined capacitances Cx, Cy are also substantially equal. As appreciated, a reduction in stray capacitance C1 due to a drop impact results in a like reduction in combined capacitance Cx, while an increased in stray capacitance C2 results in a like increase in combined capacitance Cy.
Basically, an increase in stray capacitances C1, C2 and combined capacitances Cx, Cy associated therewith results in an increase in equivalent series capacitance CQ and an eventual decrease in the oscillation frequency. Conversely, a reduction in stray capacitances C1, C2 and combined capacitances Cx, Cy associated therewith results in a reduction in equivalent series capacitance CQ and an eventual increase in the oscillation frequency. In this event, since changing amounts xcex94C1, xcex94C2 in the increasing and decreasing directions are substantially equal while associated xcex94Cx, xcex94Cy are also substantially equal, there is few change in the oscillation frequency before and after the impact because the changing amount of the increasing oscillation frequency is canceled out by the changing amount of the decreasing oscillation frequency.
Case 2
Assuming that oscillation capacitors Ca, Cb have the same value (Ca=Cb), and gap d1 is larger than gap d2 (d1 greater than d2), i.e., stray capacitance C1 is smaller than C2 (C1 less than C2), combined capacitance Cx is smaller than Cy (Cx less than Cy). In this event, for stray capacitance C1 after a drop impact, larger gap d1 causes reduced changing amount xcex94C1 in the decreasing direction and reduced changing amount xcex94Cx of combined capacitance Cx. Conversely, for stray capacitance C2, smaller gap d2 causes increased changing amount xcex94C2 in the increasing direction, and increased changing amount xcex94Cy of combined capacitance Cy. Thus, the oscillation frequency is reduced after the impact is applied because changing amounts xcex94C2 and xcex94Cy in the increasing direction are larger than changing amounts xcex94C1 and xcex94Cx in the decreasing direction, respectively.
Case 3
Assuming that oscillation capacitors Ca, Cb have the same value (Ca=Cb), and gap d1 is smaller than gap d2 (d1 less than d2), i.e., stray capacitance C1 is larger than C2 (C1 greater than C2), combined capacitance Cx is larger than Cy (Cx greater than Cy). In this event, for stray capacitance C1 after a drop impact, smaller gap d1 causes increased changing amount xcex94C1 in the decreasing direction and increased changing amount xcex94Cx of combined capacitance Cx. Conversely, for stray capacitance C2, larger gap d2 causes reduced changing amount xcex94C2 in the increasing direction and reduced changing amount xcex94Cy of combined capacitance Cy. Thus, the oscillation frequency is increased after the impact is applied because changing amount xcex94C1 and xcex94Cx in the decreasing direction is larger than changing amount xcex94C2 and xcex94Cx in the increasing direction, respectively.
When the conventional surface mounting crystal oscillator configured as illustrated in FIG. 1 was manufactured, distance d from the surface of the IC chip, i.e., the surface of the chip conductor, to metal cover 4 was approximately 0.5 mm in the thus manufactured prototype crystal oscillator. In addition, gap d1 between crystal blank 3 and IC chip 2 was significantly smaller than gap d2 between crystal blank 3 and metal cover 4 due to the structure of the surface mounting crystal oscillator. Therefore, the prototype crystal oscillator falls under the aforementioned Case 3, where there is a larger changing amount xcex94C1 of stray capacitance C1 which is a change in the decreasing direction due to gap d1 between crystal blank 3 and IC chip 2, so that it is demonstrated that the crystal oscillator presents a large change in frequency in the increasing direction after the drop impact.
A quartz crystal oscillator which does not contemplate its compact size is free from the problematic fluctuations in frequency resulting from an impact because this type of crystal oscillator has a large height and a large distance from a chip conductor to a metal cover. Specifically, the large distance between the chip conductor and metal cover results in small stray capacitances C1, C2 themselves, and a change in capacitance, if any, associated with a change in the distance caused by a drop impact would exert a negligible influence on the oscillation frequency, so that such a crystal oscillator is free from the problem as described above. Stated another way, the problem of fluctuations in frequency resulting from a drop impact is peculiar in surface mounting crystal oscillators which are intended for a reduction in size.
As described above, the present invention sets the gap between the IC chip and crystal blank, and the gap between the crystal blank and metal cover in accordance with a changing amount of the oscillation frequency due to a change in the first and second stray capacitances C1, C2 respectively connected in parallel with first capacitor Ca and second capacitor Cb in a direction in which a change in equivalent series capacitance is reduced, as viewed from the crystal unit, while maintaining a gap between the IC chip and metal cover, so that changing amount xcex94f/f of the oscillation frequency is reduced even if stray capacitances C1, C2 change after a drop impact. In addition, the crystal oscillator can maintain a small height because the spacing between the IC chip and metal cover is maintained constant.